Correction of human vision is centered, in general, upon clinical refraction, an approach based upon optics, physiology, and the psychology of perception. Generally, any refractive analysis of the human eye has some basis in optics. For example, the treatment of defective vision will consider the position of focus of the eye which may be displaced from the emmetropic retina under conditions of either myopia or hyperopia. In addition, the eye may be astigmatic, exhibiting different focal aspects for each primary meridian which, in turn, may be oriented anywhere within a 180.degree. aspect. Thus, the optometric clinician often is called upon to approach the optical aspect of diagnosis by evaluating the dioptric aspects of focal deficiency as they may be related to meridial power variances. The correction of occular astigmatism is carried out by collapsing the interval of Sturm with cylinder lenses. See the following publication in this regard: "Visual Optics and Refraction--A Clinical Approach" by D. D. Michaels, 2d Ed., C. V. Mosby Co., St. Louis, Mo. 1980.
Commonly, an ophthalmic instrument referred to as a refractor is employed for efficiently carrying out optical analysis. Refractors typically are fashioned comprising right and left batteries, each having an eye position for the patient before which any of a broad variety of disk-mounted testing lenses may be positioned. These lenses may be spherical, exhibiting a broad range of powers, or cylindrical, again exhibiting power variations but with respect to alignment along + and - axes.
Where an evaluation of the astigmatic eye is at hand, a broad variety of analytic approaches have been developed. Linksz has described a method for determining meridial orientation, i.e. by checking cylinder for axis and amount by rotating a correcting cylinder before the eye. See the following publications in this regard: Linksz, "A Determination of Axis and Amount of Astigmatic Error by Rotation of Trial Cylinder", Archives of Ophthalmology, October 1942. The rotating cylinder approach to this form of analysis was further developed into a test known as the "Jackson Cross Cylinder Test" which has been implemented broadly in ophthalmic refractors. The test is performed in both a cylinder axis and cylinder power mode. Under the test procedure, the patient is seated in a darkened examination room before the refractor and is asked to observe an illuminated distant target. The correcting cylinder axis before an appropriate eye then is manipulated by manually turning an axis control knob which is operated in conjunction with two complementary large surrounding protractor scales. Such manipulation adjusts the position of the axis of the pertinent test cylinder, its orientation is read at the scale in degrees ranging from 0.degree. to 180.degree.. Typically, the gradiations of the scales are arranged in steps of 5.degree..
Upon the axis control knob being adjusted to a first approximation, a cross-cylinder, provided as a lens consisting of equal power + and - cylinders with their axes 90.degree. apart, is positioned at the eye station. This test lens is mounted in its loupe for rotation about a "flip" axis midway between the + and - axes. When the lens is flipped, the + and - axes change places. For this axis mode testing, the cross cylinder also is positioned with respect to a first approximation such that its axis is oriented 45.degree. with respect to the correcting cylinder axis. Such aligning procedure is carried out somewhat semi-automatically. Generally, the refractor will carry the cross cylinder lens within a turret which is manually rotated to position the lens before the eye station or tube.
As the test continues, the cross cylinder lens is "flipped" from its first position to the alternate transverse position and the patient is asked which position is better. Depending upon the response and assuming testing is carried out with minus cylinder lenses, the correcting cylinder axis knob is manipulated to rotate the correcting cylinder toward the position at which vision is improved. These steps are repeated until a final end point is reached such that when the cross cylinder lens is flipped from one position to the other, the patient's vision is equally blurred. The practitioner then records the reading of the axis control knob by observing a painted line indicia thereon as it is positioned adjacent to a line of the earlier-described scale. Generally, the practitioner interpolates the axial orientation in degrees within 5.degree. steps of the scale. During the procedure of gradual refinement of axis positions, the practitioner is repeatedly called upon to reference the axis scale under less than desirable ambient lighting conditions. Generally, only when the testing is completed can the light level be raised. At times, penlights have been put to use to read scales, usually only at test completion.
following the axis mode check, the cross-cylinder lens is rotated by the operator 45.degree. to another mechanical detent control position for carrying out a cylinder power mode check. As the patient monoccularly fixates upon the illuminated target, the cross cylinder lens is flipped between alternate positions and the patient is asked, as before, at which position vision is better or worse. If vision is less blurred, correcting minus cylinder power is increased. If vision is better, the correcting cylinder power is reduced. Finally, an end point is obtained wherein correcting cylinder power is correct and the vision of the patient equally is impaired when the cross cylinder lens is flipped between its alternate positions. Readings throughout the procedure again are carried out by the practitioner under less than desirable lighting conditions.
In general, as the practitioner carries out hours of analysis with the refractor, fatigue factors and the like will set in which may lead to human error in the reading of scale based data. This, in turn, will result in lengthier tests. The provision of illuminated readouts for refraction has been suggested, for example, in U.S. Pat. No. 4,523,822 where digital readouts are provided to the practitioner through the utilization of multi-component light emitting diodes. Other attempts at improving readout have been through the uses of small pieces of plastic associated with small lightbulbs. However, the form of illumination provided by such devices has been of minimum value. In this regard, it is important, for example, that the entire axis scale be readable and that the amount of illumination supplied be as minimal as possible while remaining effective to achieve accurate recordation. Finally, it is desirable that all critical dials and readout windows be provided with efficient illumination without resort to a multitude of lighting devices and the like otherwise complicating the refractor structure.